Carbon nanoparticles having a layered configuration are known as fullerene nanoparticles. Generally, there are three main types of fullerene-related carbon particles: fullerenes (C60, C70, etc.); nested-fullerene nanoparticles (in the form of onions), and nanotubes. Analogous fullerene-like nanoparticles can be obtained from a number of inorganic materials with layered structure, and are known as inorganic fullerene-like materials.
Inorganic fullerene-like (abbreviated hereinafter “IF”) nanoparticles and nanotubes are attractive due to their unique crystallographic morphology and their interesting physical properties.
Layered transition-metal dichalcogenides MS2 (such as WS2 and MoS2) are of great interest as they act as host lattices by reacting with a variety of guest atoms or molecules to yield intercalation compounds, in which the guest is inserted between the host layers. Accordingly, IF transition metal dichalcogenides may be used for instance, for hydrogen storage.
Furthermore, disulfides of molybdenum and tungsten belong to a class of solid lubricants useful in vacuum, space and other applications where liquids are impractical to use. IF nanoparticles can be used as additives to various kinds of oils and greases to enhance their tribological behavior1. Furthermore, different coatings with impregnated IF nanoparticles were shown to exhibit self-lubricating behavior.
IF nanoparticles may also be used for other possible applications such as battery cathodes, catalysis, nanoelectronic and magnetic information storage.
The first closed-cage fullerene-like nanoparticles and nanotubes of WS2 were obtained via sulfidization of thin films of the respective trioxides in 1992, followed by MoS2 and the respective diselenides.
Numerous IF nanostructures have been synthesized using different metodologies. The first report related to IF-MS2 (IF-NbS2) structures obtained by the reaction of the metal chloride (NbCl5) and H2S2. Later on, Jun Chen et al.3 used a low-temperature gas reaction to synthesize TiS2 nanotubes. The reaction involved heating TiCl4, H2, and H2S inside a horizontal furnace at a relatively low temperature of 450° C., and in the absence of oxygen and water.
Another method and apparatus for preparing inorganic fullerene-like nanoparticles of a metal, e.g. transition metal chalcogenide having a desired size and shape in high yields and macroscopic quantities, is described in WO 97/442784. This method utilizes (a) dispersing solid particles of at least one non-volatile metal oxide material having the preselected size and shape; and (b) heating the solid particles of the non-volatile metal material in a reducing gaseous atmosphere containing at least one chalcogen material for a time and a temperature sufficient to allow the metal material precursor and the chalcogen material to react and form at least one layer of metal chalcogenide, the at least one layer of metal chalcogenide encaging the surface of the solid particles to form the fullerene-like particles.
The synthesis of IF-WS2 involves a solid-gas reaction, where the nanocrystalline tungsten oxide, serving as a precursor, reacts with H2S gas at elevated temperatures5. In a different procedure, IF-MoS2 nanoparticles are prepared in the gas phase, upon in-situ reduction and condensation of the MoO3 vapor and subsequent sulfidization by H2S6.
The availability of fullerene-like MoS2 and WS2 nanoparticles in large amounts paved the way for a systematic investigation of their properties. Both IF-WS2 and IF-MoS2 nanoparticles were found to provide beneficial tribological behavior under harsh conditions1, suggesting extensive number of tribological applications for these nanoparticles, eliciting substantial industrial interest.
Mass production of IF-WS2 was enabled by the construction of first a falling bed and subsequently fluidized bed reactors7.
Reactors for mass production of IF-WS2 and IF-MoS2 are described in WO 01/66462 and WO 2/34959, respectively8,9.
The reported IF-WS2 and IF-MoS25-7 were synthesized from their corresponding oxide crystallite that served as a template for the growth of the sulfide nanoparticles. The growth of the sulfide layers in each particle starts on the top surface of the partially reduced oxide nanoparticle terminating in its core. This diffusion-controlled reaction is rather slow, lasting a few hours. The final nanoparticles consist of dozens of sulfide layers and a hollow core occupying 5-10% of the total volume of the nanoparticles.
In another research, large-scale MoS2 and WS2 IF nanostructures (onion-like nanoparticles and nanotubes) and three-dimensional nanoflowers were selectively prepared through an atmospheric pressure chemical vapor deposition process from metal chlorides (e.g. MoCl5 and WCl6) and sulfur10. In this technique, selectivity was achieved by varying the reaction temperature, with 750° C. favoring the nanotubes and 850° C. the fullerene-like nanoparticles.
In a further research, tungsten diselenide closed-cage nanoparticles were synthesized by the reaction of prevaporized Se with WO3 powder in a reducing atmosphere11. The selenium vapor was brought to the main reaction chamber by a carrier gas. The growth mechanism of the IF-WSe2 nanoparticles was outside-in. This growth mode is analogous to the previously reported growth of IF-WS2 using the reaction between WO3 nanoparticles and H2S gas5.